1. Field of the Invention
This invention relates to an image forming method and apparatus for aiding detection of an abnormal pattern candidate embedded in a medical image, a computer program for causing a computer to execute the image forming method, and a computer readable recording medium, on which the computer program has been recorded. This invention also relates to an image selecting method and apparatus, an image output method and apparatus, and an image furnishing service system.
2. Description of the Related Art
In medical fields, operations have heretofore been performed, wherein at least two medical images of a certain patient, which medical images have been recorded at different points of time, are compared with each other, a difference between the medical images is found, and an abnormal pattern is detected in accordance with the thus found difference. Also, in accordance with the thus found difference, a state of progress of a disease or a state of cure of a disease is ascertained, and a plan of medical treatment is determined.
As a technique for detecting an abnormal pattern by comparing medical images with each other, a technique for detecting an abnormal pattern, such as a lung cancer in its early stage, which abnormal pattern is small in size and cannot be found easily, has been proposed in, for example, Japanese Unexamined Patent Publication No. 2002-158923. With the proposed technique, a difference between two medical images, which have been recorded at different points of time, is calculated with, for example, a subtraction process, in which image signal components representing corresponding pixels in the medical images are subtracted from each other. Also, a region embedded in a difference image representing the difference, i.e. a temporal (time difference) subtraction image, which region is associated with pixel values larger than a predetermined value or has a characteristic shape, e.g. an approximately circular shape, is detected as an abnormal pattern candidate.
As for the two time series images, which have been recorded at different points of time and are to be compared with each other, a shift of an object in the patient occurs due to, for example, a change in orientation of the patient at the time of the image recording operation. Therefore, in the aforesaid technique for detecting an abnormal pattern, ordinarily, position matching processing is performed on the two images, which are to be compared with each other.
As techniques for performing the position matching processing, a technique for performing global position matching processing (a linear transform, such as an affine transform), a technique for performing local position matching processing, and a technique for performing a combination of the global position matching processing and the local position matching processing have been proposed in, for example, U.S. Pat. Nos. 5,359,513, 5,790,690 and 6,011,862. With the global position matching processing, a rotating processing, parallel translation, and image size enlargement or reduction processing are performed on an entire image area of at least either one of the two images of a single object, which images are to be compared with each other. With the local position matching processing, nonlinear strain transform processing, i.e. warping, (e.g. nonlinear strain transform processing utilizing curve fitting with two-dimensional polynomials) is performed in accordance with corresponding position relationship, which has been obtained from template matching performed with respect to each of local area limited regions. Also, a technique for performing position re-matching processing with respect to a local area limited region, which is associated with a high degree of shift, and regions neighboring with the local area limited region has been proposed in, for example, Japanese Unexamined Publication No. 2002-32735.
However, with the aforesaid techniques for performing the position matching processing, the position matching processing is performed on the images, which have been projected in two-dimensional directions. Therefore, in cases where a large three-dimensional fluctuation (such as forward inclination, backward inclination, or lateral turning) occurs due to, for example, a change in patient orientation between the time series images, the problems occur in that the position matching cannot be achieved accurately, an artifact occurs in the difference image, and an image appropriate for the detection of an abnormal pattern cannot be obtained.
Also, in cases where a medical image of a certain patient had not been recorded in the past, or in cases where a medical image of a certain patient, which was recorded in the past, is not available, the problems occur in that, since the image to be compared with the newly recorded image cannot be obtained, the technique for image comparison (particularly, the technique for detecting an abnormal pattern by use of the difference image), which has the effect of detecting a small-sized abnormal pattern that cannot ordinarily be found easily, cannot be utilized.